Cokeworth is a fictional town in the English Midlands where Harry spends a night at the Railview Hotel with his aunt, uncle and cousin Dudley. Cokeworth’s name is supposed to suggest an industrial town, and to evoke associations of hard work and grime.
Although it is never made explicit in the books, Cokeworth is the place where Petunia and Lily Evans and Severus Snape all grew up. When Aunt Petunia and Uncle Vernon are trying to evade the letters from Hogwarts, they travel to Cokeworth. Perhaps Uncle Vernon has a vague idea that Cokeworth is so distinctly unmagical, the letters will not follow them there. He ought to have known better; after all Petunia’s sister, Lily, turned into a talented witch in Cokeworth.
It is therefore Cokeworth that Bellatrix and Narcissa visit at the start of Half-Blood Prince, where they visit Snape at his parents’ old house. Cokeworth has a river running through it, evidence of at least one large factory in the long chimney overlooking Snape’s house, and many small streets full of workers’ houses.